1. Technical Field
The present invention relates to anisotropic magnet powders which consist of rare earth element, iron and boron and also relates to its production method.
2. Prior Art
RFeB type rare earth magnet which is mainly composed of rare earth element including yttrium (Y), iron (Fe) and boron (B) is widely used in industry due to excellent magnetic properties. There are three patents related to the present invention, namely, Japanese Patent Application Laid-open (Kokai) No.60-257107, 62-23903 and Japanese Examined Patent Publication (Kokoku) No.7-68561.
Japanese Patent Application Laid-open (Kokai) No.62-23903 discloses production method characterized by hydrogen heat treatment at an elevated temperature, for example at 800.degree. C., which produces a RFeB type permanent magnet powder having intrinsic coercivity (iHc) of 398 kA/m. The hydrogen heat treatment comprises hydrogenation accompanied by phase transformation from R.sub.2 Fe.sub.14 B phase to RH.sub.2 +.alpha.Fe+Fe.sub.2 B and desorption accompanied by reverse transformation from RH .sub.2 +.alpha.Fe+Fe.sub.2 B to R.sub.2 Fe.sub.14 B phase. In No.62-23903, crushing step is performed between the hydrogenation step and desorption step. It had been known before Japanese Patent Application Laid-open (Kokai) No.62-23903 that phase transformation takes place in hydrogen heat treatment at an elevated temperature of 800.degree. C.
No.62-23903 is the first patent application that disclosed that the phase transformation during hydrogen heat treatment gives improved magnetic properties for RFeB type magnet alloy.
Japanese Examined Patent Publication (Kokoku) No.7-68561 discloses a production method, similar to No.62-23903 above mentioned, which produces a RFeB type permanent magnet powder having intrinsic coercivity (iHc) of 795 kA/m by omitting the crushing step to perform hydrogenation and desorption successively. In the improved hydrogen heat treatment the disproportionation reaction between RFeB type alloy and hydrogen is introduced at a temperature from 500.degree. C. to 1000.degree. C. at the hydrogen pressure of more than 1.3 kPa, and subsequent desorption reaction is introduced at the same temperature at the hydrogen pressure of less than 0.1 Torr.
No.7-68561 discloses the hydrogen heat treatment method also can introduce small degree of anisotropy in the magnet powder with the composition of Nd.sub.12.0 Pr.sub.1.4 Fe.sub.80.8 B.sub.5.8. The powder is heated to 830.degree. C. in hydrogen atmosphere at a pressure of 1 atm, held at the said temperature and pressure of 1.3 kPa-0.1 MPa for 5 hours, then held at the temperature of 830.degree. C. at reduced pressure of 1.31.times.10.sup.-3 Pa for 40 minutes, followed by quenching. The residual induction (Br) of the bonded magnet is improved from 0.61T to 0.72T, which means improvement of 18.2%, by applying magnetic field during compression-molding.
Japanese Examined Patent Publication (Kokoku) No.4-20242 discloses a production method for rare earth permanent magnet having high degree of anisotropy by aligning the crystal orientation of microstructure in the rare earth magnet by hot deformation of melt-spun rare earth material.
The Problem to be Solved by the Invention
It is the object of the invention to provide a rare earth magnet powder having excellent anisotropy, that means Br/Bs of more than 0.65, by hydrogen heat treatment.
Thus far, two major methods, hot deformation process and hydrogen heat treatment, are proposed to introduce anisotropy in the magnet powder.
The hot deformation process that introduces alignment of the crystal orientation of microstructure in the rare earth magnet has a drawback of increased production cost because of its complicated process. In addition to said drawback, the grain of the powder is plate-like which is unfavorable for the production of bonded magnet.
On the other hand, hydrogen heat treatment is known to improve magnetic properties such as magnetic remanence and coercivity. In the hydrogen heat treatment, the grain size is refined by the phase transformations introduced during the hydrogenation process and the hydrogen desorption process and thus enhances magnetic properties. This hydrogen heat treatment has a merit of low production cost due to relatively simple operation, however, the obtained magnetic properties are not satisfactory. It is also quite difficult to obtain anisotropic magnet powder by this treatment.
As is mentioned previously, in Japanese Examined Patent Publication No.7-68561, it is reported that the Br of the Nd.sub.12.0 Pr.sub.1.4 Fe.sub.80.8 B.sub.5.8 rare earth magnet is improved from 0.61T to 0.72T, which means improvement of 18.2%, by applying magnetic field during compression-molding.
One of the inventors of the patent application insists in his review in J. Alloys and Compounds 231 (1995) 51 that the substitution of Fe with Co in the ternary Nd--Fe--B alloy and additions of elements such as Zr, Ga, Hf, Nb are essential to obtain high degree of anisotropy, but the ternary Nd--Fe--B alloy without substitution of Co only gives isotropic magnet even though it is hydrogen-heat treated.
By an intensive study for hydrogen heat treatment of RFeB magnet, we have discovered that the ternary NdFeB alloy without any substitution nor addition of elements, considered to be isotropic thus far, can have high degree of anisotropy by hydrogen heat treatment. In other words we have discovered that the residual induction (Br) is improved from 0.8T to 1.2-1.5T in NdFeB ternary alloy without Co substitution by the hydrogen heat treatment.
We consider that anisotropy of the magnet is induced in following way. First, the crystal orientation of Nd.sub.2 Fe.sub.14 B is transferred to the crystal orientation of nanocrystalline Fe.sub.2 B that is generated by the phase transformation during the hydrogenaration of NdFeB alloy in hydrogen heat treatment. Then, the crystal orientation of nanocrystalline Fe.sub.2 B is transferred to the crystal orientation of nanocrystalline Nd.sub.2 Fe.sub.14 B that is regenerated by the reverse transformation during hydrogen desorption in hydrogen heat treatment. As a result the original orientation of mother crystal can be kept in the nanocrystalline Nd.sub.2 Fe.sub.14 B after phase transformations, that is, the obtained magnet powder shows very high degree of anisotropy.
The present invention is completed from the viewpoint described above.
Means of Solving the Problem
The rare earth magnet powder offered by present invention consists essentially of rare earth element including yttrium (hereinafter denoted by R), iron (Fe), and boron (B) and have excellent anisotropy, that means Br/Bs of more than 0.65, given by hydrogen heat treatment accompanied with phase transformation.
Here, the inventors define anisotropy as Br/Bs (here Bs equals to 1.6T(16 kG)) of more than 0.65, perfect anisotropy as Br/Bs of unity, isotropy as Br/Bs of over 0.5 to 0.65, and perfect isotropy as Br/Bs of 0.5, respectively. It is very hard to obtain perfect isotropy or perfect anisotropy because RFeB crystal is anisotropic by nature, on the other hand the particle in the powder is apt to be isotropic. The high degree of anisotropy in RFeB based alloy by the present invention is due to good alignment of recrystallized grains of R.sub.2 Fe.sub.14 B of which tetragonal structure, as is well known, shows strong crystal magnetic anisotropy.
Br of the specimen is measured by VSM (vibrating sample magnetometer) because ordinary BH curve tracer can not measure Br of the powder materials.
The specimens are prepared in following method. The magnet powder is classified into 74-105 .mu.m size. The powder is formed into a specimen having demagnetizing factor of 0.2, and then the magnetization direction of each particle is aligned in a magnetic field of 4578 kA/m (45 kOe).
The rare earth magnet powder offered by present invention is also characterized by its spherical grain shape from the anisotropic powders obtained by the hot deformation process. The grains of the powder according to the present invention are spherical with the aspect ratio of less than 2.0. Its diameter is 0.1-1.0 .mu.m.
Here, the word "grain" does not mean each magnet powder particle, but means each crystal grain. One particle of the powder consists of many crystal grains. The aspect ratio is defined as the ratio of the greatest dimension to the smallest dimension (greatest dimension/smallest dimension) of the grain.
The grains of rare earth magnet by hot deformation is plate-like shape, which is entirely different from the ones obtained by hydrogen heat treatment mentioned above.
The RFeB based alloy for the present invented anisotropic magnet powder is composed of 12-15at % R, 5.5-8 at % B and the balance of Fe with unavoidable impurity.
R of more than 15 at % causes decrease in Br due to decrease in magnetic moment of the crystal, conversely, R of less than 12 at % causes decrease in iHc due to increased .alpha.Fe amount in primary crystal.
B of more than 8 at % causes decrease in Br due to decrease in magnetic moment of the crystal, conversely, B of less than 5.5 at % causes decrease in iHc due to precipitation of R.sub.2 Fe.sub.17.
R can be one or more rare earth elements chosen from the group of Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu. It is desirable to choose Nd due to its low cost and potential of offering superior magnetic properties for its alloys.
Addition of 0.01-1.0 at % of Ga enhances intrinsic coercivity of RFeB. The reason is considered that Ga may smooth out the grain boundary and suppress the nucleation of the reversed magnetic domains. Addition of 0.01-0.6 at % of Nb together with the hydrogen heat treatment offered in the present invention enhances the degree of anisotropy, however, the enhancement by Nb addition is obtained only in the case that magnet powder is treated by present invented hydrogen heat treatment. In other words, the enhancement is the interaction with Nb addition and proper condition of hydrogen heat treatment. The reason of enhancement is considered that Nb may help to transfer the crystal orientation of Nd.sub.2 Fe.sub.14 B to that of Fe.sub.2 B.
The raw material of the present invented anisotropic magnet powder is prepared by melting prescribed amount of purified rare earth, iron, and boron in a furnace, and then cast into an ingot. The ingot can be crushed into powder and used as raw material.
It is desirable that the raw materials are homogenized, since homogenizing treatment decreases segregation that is unfavorable for the magnetic properties.
The raw material prepared in the foregoing manner is subjected to the hydrogen heat treatment described below to have the degree of anisotropy (Br/Bs, here Bs equals to 1.6T(16 kG)) of more than 0.65, Br of 1.2 to 1.5T, iHc of 636 to 1272 kA/m, and maximum energy product ((BH)max) of 238 to 358 kJ/m.sup.3.
The hydrogen heat treatment of the present invention is characterized by controlling the reaction between the alloy and hydrogen at the relative reaction rate of 0.25-0.50 in which phase transformation is introduced during hydrogenation and reverse transformation during hydrogen desorption in RFeB.
The inventors have found that the rate of phase transformation has great effect on the magnetic anisotropy of RFeB based alloys.
The rate of phase transformation is proportional to the rate of reaction between the hydrogen and RFeB based alloys.
The rate of reaction V is defined as; EQU V=V.sub.o .times.(PH.sub.2 /P.sub.0).sup.1/2 .times.exp(-E.sub.a /RT)
here, V.sub.o is frequency factor, PH.sub.2 is hydrogen pressure, P.sub.0 is dissociation pressure, Ea is activation energy of the alloy, R is gas constant, T is absolute temperature of the system.
The relative reaction rate V.sub.r is defined as the ratio of reaction rate V to the standard reaction rate V.sub.b, which is defined as the reaction rate for the temperature of 830.degree. C. and hydrogen pressure of 0.1 MPa in the case of phase transformation of R.sub.2 Fe.sub.14 B phase to RH.sub.2 +.alpha.Fe+Fe.sub.2 B. Therefore V.sub.r =V/V.sub.b =(1/0.576).times.(PH.sub.2).sup.1/2 .times.exp (-Ea/RT) for the phase transformation.
As for the reverse transformation, V.sub.b (reverse) is the reaction rate for the temperature of 830.degree. C. and hydrogen pressure of 0.001 MPa.
V.sub.r (reverse) for the reverse transformation can be obtained similar manner.
As shown in FIG. 1, activation energy (Ea) depends on the composition and varies from 195 to 200 kJ/mol.
The activation energy Ea for each composition is calculated value using the value of the formation heat of NH.sub.2 from Nd and Hydrogen.
The relationship between reaction temperature and relative reaction rate at Ea=195 kJ/mol, the value of which is for stoichiometric Nd.sub.2 Fe.sub.14 B, is shown in FIG. 2. In the figure the square area shows the temperature range for desirable relative reaction rate, the range of 0.25-0.5. Similarly, the relationship between hydrogen pressure and relative reaction rate at same Ea is shown in FIG. 3. In the figure the square area shows the hydrogen pressure range for desirable relative reaction rate. In order to obtain the relative reaction rate of phase transformation within the range of 0.25-0.50, reaction temperature and hydrogen pressure have to be set within 780-840.degree. C., 0.01-0.06 MPa, respectively.
When the relative reaction rate of phase transformation in RFeB is controlled within the range of 0.25-0.50, anisotropic magnet powders of more than 0.65 in Br/Bs are obtained. As mentioned before, we consider that anisotropy of the magnet is induced by transfer of the crystal orientation of Nd.sub.2 Fe.sub.14 B to the crystal orientation of nanocrystalline Fe.sub.2 B. As a result the magnet shows very high degree of anisotropy, that is, Br/Bs of more than 0.65.
When the relative reaction rate is greater than 0.50 only isotropic magnet powder is obtained. It is because that the phase transformation takes place too rapid to transfer the crystal orientation of Nd.sub.2 Fe.sub.14 B to the Fe.sub.2 B crystal in order. Thus the degree of anisotropy decreases to Br/Bs of less than 0.65.
It is found out that good transfer of crystal orientation in the phase transformation assures anisotropy of Nd.sub.2 Fe.sub.14 B crystal after the hydrogen desorption step. In other words, sufficient transfer of crystal orientation is essential for the magnet to have high degree of anisotropy.
On the other hand, when the relative reaction rate is smaller than 0.25, the high coercivity (iHc) is not obtained. (BH)max is not high although Br is relatively high. It is because that the phase transformation from Nd.sub.2 Fe.sub.14 B phase to RH.sub.2 +.alpha.Fe+Fe.sub.2 B is not finished and the residual Nd.sub.2 Fe.sub.14 B makes microstructure after reverse transformation inhomogeneous.
In the foregoing the word "reaction temperature" indicates the actual temperature at which phase transformation in RFeB takes place by absorption of hydrogen. It should be noted that the reaction temperature is different from the control temperature of the furnace.
The temperature of the RFeB alloy in the reaction may be much higher than that of the furnace because the heat generated by the reaction, which is exothermic, may raise its temperature.
Similarly the hydrogen pressure may vary from the set pressure, because the pressure of hydrogen in neighborhood of alloy is apt to decrease by depletion due to occlusion of hydrogen in RFeB. Especially in the case when the atmosphere is mixture of inert gas and hydrogen, partial pressure of hydrogen in neighborhood of alloy may significantly decrease by depletion due to occlusion of hydrogen in RFeB while total pressure is kept at near the set pressure.
The above phenomena concerning the reaction between hydrogen and RFeB results difficulty in controlling the temperature and pressure for the conventional furnaces to obtain the relative rate of reaction within the range of 0.25-0.50 for following reasons. One is that the deviations in temperature and pressure are proportional to the mass of the material in the furnace. The other is that the reaction is self-exciting, i.e., it is accelerated by the heat generated in the exothermic reaction itself. Therefore, to obtain desired conditions for the reaction, specially designed furnace is needed to have proper control of temperature and hydrogen pressure.
To solve the above mentioned problem, we have developed and employed a novel furnace that can cancel the heat generated by the reaction. The furnace is disclosed in the Japanese Patent Application Laid-open (kokai) No.8-206231.
The furnace has heat absorbent means to cancel the heat generated by the reaction with hydrogen in the raw material. The furnace has a set of a processing vessel and a heat compensating vessel in contact. Raw material is inserted in the processing vessel in which the hydrogen pressure as well as the temperature can be controlled. Dummy material is inserted in the compensating vessel also in which the hydrogen pressure as well as the temperature can be controlled independently with the processing vessel. When exothermic reaction occurs and the material generates heat in the processing vessel, in addition to turning off the heater, the hydrogen pressure of the compensating vessel is decreased to start endothermic reaction and to compensate for the generated heat in the processing vessel. Thus the reaction temperature can be kept to nearly equal to the set temperature of the furnace.
As a result the desired conditions for the phase transformation can be obtained in mass production by this novel furnace.
After the completion of hydrogenation step, it is desirable to subject the powder in stress relief treatment, that is, to keep the magnet powder at same temperature. It is found out that intrinsic coercivity of the powder is enhanced by keeping the powder for at least an hour. The completion of hydrogenation requires about 30 minutes, however actually it depends on treatment volume.
The reason is considered that the enhancement in coercivity may be due to the relaxation of the internal strain induced by the phase transformation.
If the internal strain remains in magnet powder, microstructure after hydrogen desorption may be apt to inhomogeneous, and the intrinsic coercivity is deteriorate as a result.
After the stress relief treatment in hydrogen, the powder is subjected to hydrogen desorption step in which the reverse transformation takes place.
During this reverse transformation the crystalline orientation of nanocrystalline Fe.sub.2 B is transferred to the orientation of nanocrystalline Nd.sub.2 Fe.sub.14 B.
It is desirable to control the the relative reaction rate of reverse transformation within the range of 0.1-0.4 to transfer the crystal orientation of nanocrystalline Fe.sub.2 B in a low hydrogen pressure at the same temperature to hydrogenation step.
When the relative reaction rate of reverse transformation is controlled within the range of 0.1-0.4, the degree of anisotropy is enhanced by transferring of the crystal orientation of nanocrystalline Fe.sub.2 B to the crystal orientation of nanocrystalline Nd.sub.2 Fe.sub.14 B in good order.
For the relative reaction rate of reverse transformation smaller than the above range, the high coercivity is not obtained owing to inhomogeneous microstructure due to imbalance in nucleation and growth. On the other hand, for the relative reaction rate greater than the above range, the degree of anisotropy in the magnet powder is deteriorated because the transfer of the crystal orientation of nanocrystalline Fe.sub.2 B to that of recrystallized Nd.sub.2 Fe.sub.14 B is disturbed.
In the actual operation the hydrogen pressure is set 1/10-1/100 of the pressure in the hydrogenation step.
However the reverse transformation, which is endothermic, is also self-exciting, therefore the temperature of the raw material significantly decrease.
In order to set the reaction temperature within 780-840.degree. C., it is necessary to use the same furnace for the hydrogenation process.
The time needed to complete the reaction of the reverse transformation is supposed to be about 10 minutes, however actually it depends on treatment volume.
After the completion of reverse transformation, hydrogen which comes by NdH.sub.2 decomposition remains in the Nd.sub.2 Fe.sub.14 B crystal lattice. Hydrogen elimination treatment is carried out in which the powder is kept at same temperature for a certain time. The treatment is to perfectly eliminate the remaining hydrogen in the Nd.sub.2 Fe.sub.14 B crystal lattice. It is found out that intrinsic coercivity is enhanced by at least 25 minutes of hydrogen elimination treatment. It is considered that remaining hydrogen in the lattice deteriorates intrinsic coercivity of Nd.sub.2 Fe.sub.14 B powder.
After the hydrogen elimination treatment the powder in the furnace is cooled to the room temperature. Cooling should be done as rapid as possible, at least 5.degree. C./min is needed. Only by following forgoing steps the anisotropic magnet powder with Br/Bs of more than 0.65 is obtained.
The obtained powder is formed into bonded magnet by following conventional process.
As the raw material ingot or powder is used. The processed material will be small lumps. They can be pulverized easily into anisotropic magnet powder by the mortar or crushing machine.
The anisotropic bonded magnet is produced from compound of anisotropic magnet powder and binder. As the binder resin, low melting point metals, rubber and so on are available. When thermosetting resin such as epoxy resin is selected as binder, compression molding is employed. First the magnet powder and the thermosetting resin are mixed into compounds. Then compression molding was carried out with applied magnetic field, then the magnet is cured by heat treatment.
The present invented anisotropic magnet powder has very high degree of anisotropy, evaluated by Br/Bs, of more than 0.65. It has a superior magnetic properties, that is, Br of over 1.2T (12 kG) and iHc of over 636 kA/m (8 kOe). The anisotropic bonded magnet made from the powder has excellent (BH)max of more than 135 kJ/m.sup.3.
Above superior magnetic properties are obtained only by the present invented process in which the relative reaction rate of the phase transformation in hydrogen heat treatment is controlled within fixed rate of 0.25-0.50. The process is applicable to mass production of rare earth magnet having the excellent anisotropy mentioned above by employing the novel furnace developed by us.
Embodiment according to the present invention will be described hereafter.
The present invented anisotropic magnet powder has very high degree of anisotropy, that is, Br/Bs of more than 0.65, here Bs equals to 1.6T(16 kG).
The anisotropic bonded magnet made from the anisotropic magnet powder has excellent (BH)max.
The present invented anisotropic magnet powder can be produced by choosing proper range of phase transformation rate of hydrogenation.